The track from Wadi Mousa village to Petra passes first through a wide valley between pale sandstone hills. In Arabic it is called Bab as-Siq – gateway to the gorge – for it leads into the great natural cleft in the rock known as the Siq, the most dramatic of the entrances to Petra. Here in early Nabataean times traders would arrive with their weary caravans, filling the air with clouds of dust and the vivid sounds of men shouting, camels grunting their complaints, donkeys braying, dogs barking. Here they would settle for a while, unloading their cargoes of pungent spices, myrrh, frankincense, precious stones and rich fabrics.

As the Bab as-Siq narrows, three curious monuments stand solid sentinel, carved from the rock in the form of square towers. They are known in English as Djinn Blocks, by fanciful association with the djinn, malevolent spirits of Arab folklore...

The Arabic name for the djinn blocks, sahreej, is more prosaic for it simply means 'cistern' – which they are not. They are tower tombs, their shape suggesting that they may also have been symbols of the god Dushara, who in early Nabataean times was commonly represented as a block of stone. While they are believed to be among the earliest tombs in Petra, their date is unknown. Twenty six djinn blocks have been found in and around Petra.

The Snake Tomb

A little further along, on the other side of the road, is a curiosity that is rarely visited, though now steps have been built up to it. The Snake Tomb is carved into the rock with a narrow entrance and no outward display, but inside 12 graves are cut into the floor – clearly a family affair. On one wall there is a rough relief carving of two snakes attacking a four-legged animal (a dog?), while above it to the left is a horse with a block-shaped rider, carved on a smaller scale. What the figures signify is a puzzle, but the snake is believed to be a representation of the guardians of the underworld.

Dominating this southern side of the road is the striking carved façade of the Obelisk Tomb, and immediately below it the Bab as-Siq Triclinium. (from the Latin triclinium, pl. triclinia, which means 'three benches'). The latter is one of many such rooms in Petra which were used for memorial feasts in honour of the dead. The four magisterial obelisks across the top of the tomb smack of Egyptian stylistic ideas, while the triclinium, with its broken pediment, is more in the classical Nabataean style.

As with so much in Petra, it is unclear when these monuments were carved. Some scholars suggest the tomb is older than the triclinium; others date both to the mid-first century AD. Some believe that the bilingual inscription on the rock-face on the other side of the road, located with all the subtlety of a roadside advertisement, refers to both monuments. The longer Nabataean version reads: 'This is the burial place chosen by 'Abdmank, son of 'Akayus, son of Shullay, son of 'Utaih, for the construction of a tomb for himself, for his heirs and the heirs of his [heirs], for eternity and beyond: [he has made it] in his lifetime, in year ... of Malichus'. The Greek version is simply a summary: 'Abdomanchos son of Achaios has made this monument for himself and his children'.

The Dam

Some distance beyond the Obelisk Tomb the Bab as-Siq turns right and appears to come to an abrupt end. Here the ground rises and at the top is the narrow entrance to the Siq on the left, and ahead a tunnel cut through the mountain, with daylight visible at the end. This raised area is the Dam, probably built in the first century BC when the Nabataeans were developing Petra as their capital, and they wanted to prevent the waters of Wadi Mousa in their winter spate from cascading down the Siq in a raging torrent. The dam was rebuilt after a flash flood in 1963, and again after exceptionally heavy rains in March 1991.

To divert the waters they had to cut an 88-metre-long tunnel so that the water flowed right around the great al-Khubtha mountain via Wadi Muthlim and Wadi Mataha, rejoining the main course of Wadi Mousa by the Nymphaeum. It was a magnificent piece of hydraulic engineering – but only a fraction of the complex system that the Nabataeans created as the population of Petra increased, and their agricultural production expanded. To meet their enlarged needs, they diverted the water of the springs in the surrounding mountains and, with a skilful combination of reservoirs, dams, cisterns, channels and even pressure pipes, they increased the controlled flow of water into the very heart of the city and to their gardens.